David Cameron has been making contingency plans for a limited cabinet reshuffle as the Crown Prosecution Service disclosed that it will announceon Friday whether it will charge the energy secretary, Chris Huhne, over allegations he avoided a speeding penalty.

Keir Starmer, the director of public prosecutions, will give his decision on Huhne, and his former wife, Vicky Pryce, in a televised announcement at 10am.

Huhne has let it be known that if he is prosecuted he will declare his innocence but resign his cabinet post to fight the case, admitting the battle to clear his name would be too much of a distraction to continue in office.

Nick Clegg, the Liberal Democrat leader, has made it clear his cabinet members must be seen to behave with the highest standards of probity.

Huhne was at a Lib Dem awayday in Eastbourne when he heard the news of the CPS announcement, and left to prepare himself at his London flat. He and Pryce will be told of the decision about an hour before it is announced publicly.

The cabinet secretary, Sir Jeremy Heywood, will be told, to give the civil service time to finalise contingency plans for a reshuffle. Cameron was rearranging a regional tour to be able to take swift decisions.

In an interview with House magazine released on Thursday night, Clegg said he did not know if Huhne would be prosecuted. He said: "I don't have a crystal ball. All I can tell you is that he has made it very clear to me privately and he's said it publicly that he denies any kind of wrongdoing."

Lib Dem sources insisted they had no insight into the CPS decision, but if Huhne is prosecuted, it will largely be for Clegg to tell Cameron who he would like to succeed Huhne.

It would be the second enforced departure of a Lib Dem from the cabinet since the coalition came to power. David Laws resigned as Treasury chief secretary over expenses allegations.

In his interview Clegg praised Laws and said he would like him near the centre of power. "I'm not wildly hierarchical and David certainly isn't. It's one of the many things which I like so much about David, he's a sort of an unusual combination of being a politician but actually quite a modest character, which you don't find very often in politics. David is not after status."

But on Thursday night the party was indicating that Laws will not rejoin the government. Instead, he has adopted a backroom role for Clegg, fixing issues across the government.

If Huhne does resign, it will mean the loss of one of the party's heavy hitters, prepared to stand his ground in public against the Conservatives.

He has fought a rearguard action in Whitehall to keep green politics at the centre of government thinking at a time when the recession has made climate change a more difficult cause to sell.

Huhne has crossed swords with a succession of Tory cabinet ministers, including the chancellor, George Osborne, the Conservative co-chairman, Lady Warsi, and the home secretary, Theresa May.

Few tears will be shed in parts of Downing Street if he is prosecuted, even though Cameron is temperamentally opposed to reshuffles.

There has been repeated Conservative-led speculation that the business minister responsible for employment relations and the Post office, Ed Davey, would be given Huhne's job. Vince Cable, the business secretary and a former Shell economist, has been interested in the energy post in the past.

Since July, the CPS has been assessing evidence assembled by Essex police over whether Huhne asked his then wife to take speeding points on his behalf. The police began an investigation after claims emerged that Pryce had allegedly accepted speeding penalty points relating to an incident in 2003 that should have been given to her husband.

The alleged speeding offence is said to have taken place when Huhne was driving from Stansted airport after flying back from the European parliament, where he was an MEP at the time. If convicted of a speeding offence, he would have lost his licence.

Huhne, who left Pryce, a successful economist, for Carina Trimingham in 2010, has denied any wrongdoing.

Last month it was reported that Essex police had recommended that Huhne and Pryce be charged in the files they passed to the CPS.

It was reported that detectives believed they had given the CPS enough evidence to charge the energy secretary.

Police delivered an initial evidence file to the CPS in July, but a decision on charges was held up by legal proceedings in which the police attempted to obtain evidence from the Sunday Times.

Essex police obtained a production order to seize emails between Pryce and Isabel Oakeshott, the Sunday Times's political editor, but lawyers for the paper decided to seek a judicial review of the move.

Earlier this month, the newspaper dropped its challenge and said the production order would be complied with soon. It is understood police visited Oakeshott shortly after the hearing.

The CPS has to believe it has a more than 50% chance of conviction to proceed, and it has to be in the public interest to prosecute.

Police have already had access to a tape in which Pryce appears to discuss with Huhne the way in which he allegedly asked her to take the speeding points on.

Huhne has said he welcomed the police investigation as it would "draw a line under the matter". He and Pryce divorced in January last year.

• This article was amended on 3 February 2012. In the original the interview with Chris Huhne was attributed to Politics Home. This has been corrected.